Project Summary Bullous pemphigoid (BP) is an autoimmune subepidermal blistering disease characterized by autoantibodies and an inflammatory infiltrate at the lesional site. BP autoantibodies belong to IgG and IgE isotypes and recognize two hemidesmosomal proteins, BP180 and BP230. These autoantibodies mainly target the NC16A extracellular domain of BP180. The dermal infiltrate contains eosinophils, neutrophils, mast cells, macrophage and lymphocytes, with eosinophils being the predominant cell type. The roles of anti-BP180 IgG autoantibodies and neutrophils in BP have been extensively investigated. In vitro and in vivo studies have demonstrated that anti-BP180 NC16A IgG autoantibodies fix complement, recruit neutrophils and cause BP-like blisters. However, roles of anti-BP180 IgE and eosinophils in BP remain largely unknown. Lack of a suitable animal model is a major obstacle for studies of IgE and eosinophils because human IgE do not recognize mouse IgE receptors. Our lab currently developed a double humanized mouse stain (termed hFc?RI/NC16A mice), in which the human NC16A domain replaced the mouse counterpart NC14A domain and human Fc?RI are expressed on eosinophils and mast cells. More significantly, adult hFc?RI/NC16A mice injected with anti- NC16A IgE autoantibodies from BP patients develop BP skin disease that required eosinophils and mast cells. The objective of this proposal is to study the role of eosinophils and mast cells in BP using our newly developed hFc?RI/NC16A mouse model. The overall goal of this project is to increase our understanding of the innate immunity of BP and how it relates to the functions of innate immune system players in inflammation and autoimmunity. In this grant proposal, we will determine how eosinophils are recruited and functionally interact with mast cells (Aims 1 and 2). Proteolytic enzymes from these innate immune cells are critical for skin tissue injury, therefore, we will translate our animal model findings to human BP (Aim 3). Since this proposal integrates both disease mechanism studies and preclinical trials, the findings are expected to have a significant impact on the treatment of patients with BP.